With reference to prior art FIG. 1, in some ground based satellite communication antenna systems 100, a single antenna (feed horn) 120 is connected to a transceiver 101, where the transceiver combines the functionality of both a transmitter and a receiver. In these embodiments, typically, the transceiver has a transmit port and a receive port. The transmit and receive ports are connected to an antenna feed 105. Antenna feed 105 generally comprises an orthomode transducer (OMT) 130, a polarizer 110, and feed horn 120.
The feed horn, in this satellite communications antenna system arrangement, is a component that can convey RF signals to/from a remote location, such as a satellite. Feed horn 120 is connected to polarizer 110 and communicates transmit and receive radio frequency (RF) signals between the polarizer and the feed horn. Typically, signals communicated between feed horn 120 and polarizer 110 are circularly polarized. Polarizer 110 is configured to convert linearly polarized signals to circular polarized signals and vice versa. Thus, in linearly polarized systems, a polarizer is not required and feed horn 120 connects directly to OMT 130. Although described as two signals, the linearly polarized signals and circular polarized signals are communicated through a single port of polarizer 110 to a common port of OMT 130. Moreover, the transmit and receive signals remain isolated due to at least one, or any combination of, polarization, frequency, and time diversity.
Antenna systems for satellite communications may be configured to operate in two distinct frequency band segments where a first band segment is used to receive signals on a forward link and the second band segment is used to transmit signals on a return link from the satellite. Signals and information on each of the frequency band segments may be contained in single or dual orthogonal polarizations. Moreover, the orthogonal polarizations may be used to isolate the signals to increase capacity through frequency reuse. Military and commercial satellite systems may operate in the high frequency spectrum of frequencies known as K-band and Ka-band, which are about 20 GHz and about 30 GHz, respectively. A typical satellite antenna system operating in K/Ka-band may be configured to transmit and receive using circular polarization and may have opposite sense polarizations as one method of isolating signals in the system. For example, a transmit signal may be on a right hand circular polarization and a receive signal may be on the orthogonal left hand circular polarization sense. The quality of the circular polarization is an important factor in signal isolation. A high degree of circularity or low axial ratio in the antenna system equipment, namely the antenna optics and the RF feed components, increases the polarization performance characteristics and net system performance.
With momentary reference to prior art FIG. 1, OMT 130 may be external to transceiver 101. In addition to the common port, OMT 130 further comprises a transmit port and a receive port that are connected to matching ports on the transceiver housing. Thus, OMT 130 serves as a waveguide configured to connect a common port with at least a transmit port and a receive port. The common port may support two orthogonal polarizations. Furthermore, the common port may support two orthogonal polarizations in two distinct band segments, such as K/Ka-band. The OMT acts as a combiner/splitter of an RF signal so that a receive signal and a transmit signal can be communicated through the same feed horn with orthogonal polarizations.
The use of dual-circular polarization may present additional requirements on the feed system due to the operational nature of circularly polarized signals. Circularly polarized signals change sense or become the opposite polarity upon reflection from an impedance mismatch or discontinuity along the RF signal path. The single or multiple reflected circular polarization signals in a constrained or guided RF signal path can have deleterious effects on system performance in systems that use polarization to isolate signals. Multiple reflected signals may degrade the polarization performance of a co-polarized, or same sense polarization, signal through an interference effect. Single or multiple reflected signals may degrade the isolation to a cross-polarized, or opposite sense polarization, signal through a coupling effect.
Although this satellite antenna system is successfully employed in many systems, a need exists for high performing antenna systems that address issues of cost, ease of assembly, robustness, and tight manufacturing tolerances and the like due to operation at high frequency bands such as K/Ka-band.
First, there is a need in a dual band antenna system operating with dual-circular polarization to terminate unwanted signal reflections to eliminate or minimize multiple reflections that may degrade the polarization quality. Moreover, the dual-band four-port OMT needs tight manufacturing tolerance values for high frequency operations in order to achieve good performance. Thus, it is desirable to have an OMT that is amenable to high volume, low cost manufacturing techniques and that is robust and achieves high performance. More specifically it is desirable to have a dual-band four-port OMT that can be molded or cast in as few as two pieces.
Thus, a need exists for improved satellite antenna systems, methods and devices for addressing these and other issues.